The present invention relates to an optical isolator serving as a unidirectional waveguide for preventing light reflected by various optical devices from being applied to a semiconductor laser used as light source in optical communications, optical disc systems, or the like, thereby to stabilize excitation of the semiconductor laser.
Thin-film optical isolators having sufficient characteristics have not yet been achieved so far. One conventional two-region optical isolator is illustrated in FIG. 8 of the accompanying drawings. The two-region optical isolator generally comprises a substrate 81 made of GGG (Gd.sub.3 Ga.sub.5 O.sub.12) or the like, a magnetic thin film 82 formed on the substrate 81 of YIG (Y.sub.3 Fe.sub.5 O.sub.12), Bi: YIG (Bi.sub.x Y.sub.3-x Fe.sub.5 O.sub.12), Bi:GdID (Bi.sub.x Gd.sub.3-x Fe.sub.5 O.sub.12), or the like, and a pair of metallic cladding layers 83 made of Al or the like and placed on the magnetic thin film 82. Mode selectors 84 including the matallic cladding layers 83 largely attenuate a light beam in a TM mode, but only passes a laser beam in a TE mode. A mode converter 85 comprises a non-reciprocal region 86 and a reciprocal region 87. The magnetic thin films in these regions are magnetized respectively in a direction parallel to the direction in which the light beam is propagated and in a direction normal to the light beam propagating direction and inclined at .theta. to a direction normal to the film surface. The non-reciprocal and reciprocal regions 86, 87 effect 50% of non-reciprocal and reciprocal mode conversion due to the Faraday effect and the Cotton-Mouton effect, respectively. The mode conversion effected by the non-reciprocal region 86 cancels out the mode conversion effected by the reciprocal region 87 in a forward direction, and the mode conversion effected by the non-reciprocal region 86 is added to the mode conversion effected by the reciprocal region 87 in a reverse direction. More specifically, a light beam entering the optical isolator from its lefthand end (as shown) is transmitted only in the TE mode by one of the mode selectors 84. 50% of the light beam in the TE mode is then converted into a light beam in the TM mode by the non-reciprocal region 86 due to the Faraday effect. The reciprocal region 87 effects mode conversion due to the Cotton-Mouton effect to cancel out the mode conversion effected by the non-reciprocal region 86 due to the Faraday effect. Therefore, the light beam transmitted in the TM mode is converted back to the light beam in the TE mode again. Accordingly, the light beam passes through the other mode selector 84 and leaves the optical isolator from the righthand end thereof.
Conversely, a light beam that has entered the optical isolator from the righthand end is transmitted only in the TE mode by the mode selector 84 and 50% of the light beam is converted into a light beam in the TM mode by the reciprocal region 87. The remaining 50% of the light beam in the TE mode is further Converted into a light beam in the TM mode by the mode conversion in the non-reciprocal region 86. Therefore, since the light beam transmitted only in the TE mode from the mode selector 84 is entirely attenuated, and no light beam is discharged from the lefthand end of the optical isolator.
Other conventional optical isolators include single-region optical isolators disclosed in Electronic and Communications Society Technical Research Reports MW 86-124 (1986 written by Ueki and Miyazaki and MW 86-126 (1986) written by Taki and Miyazaki.
In the two-region optical isolator, the two adjacent non-reciprocal and reciprocal regions must be magnetized in mutually different directions. However, it is very difficult to orient the magnetization in different directions. The directions of magnetization vary complexly in the vicinity of the boundary between the non-reciprocal and reciprocal regions, and so do the magnitudes of mode conversion, with the result that no desired characteristics are obtained. Any clear data on the structural conditions and conditions for applying the magnetic field are not given with respect to the single-region optical isolators. Single-region optical isolators with sufficient characteristics have not yet been accomplished. Since the conventional optical isolators propagate light beams in multiple modes, their optical isolation capability is greatly lowered by higher-order-mode components of light that has returned to the optical isolators.